How Does MySQL Handle JOINs Without an ON Clause?

Unveiling the Mystery of MySQL JOINs Without ON Conditions

It's not uncommon to encounter scenarios where the need arises to merge data from multiple tables without explicitly specifying an ON condition in a JOIN query. In such cases, MySQL offers unique join functionality that deviates from the conventional ANSI standard. Understanding this variation allows developers to harness the power of MySQL's JOIN mechanism effectively.

Opting for a Cross Join

When utilizing JOIN or INNER JOIN without an ON clause, MySQL conducts a cross join operation. This action differs significantly from other databases and the ANSI standard, as it produces a Cartesian product. In simpler terms, it generates every possible combination of one row from each specified table.

Example:

Consider two tables, A and B, each with three and four rows, respectively. A cross join between these tables would result in 12 rows, each representing a unique pairing of rows from A and B.

Cross Join Best Practices

While cross joins can be useful in certain situations, it's crucial to use them judiciously. Using the CROSS JOIN syntax is highly recommended to explicitly indicate a cross join operation instead of relying on the absence of an ON clause. This promotes clarity and avoids potential confusion.

Right and Left Outer Joins

In contrast to cross joins, right and left outer joins require an ON clause to function correctly. Thus, the discussion about joining without an ON condition does not pertain to these types of joins.

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